1. Field of the Invention
This invention relates to techniques for the detection of toxoplasma infections and is particularly directed to methods for distinguishing acute infection from chronic infection.
2. Description of the Background
The term toxoplasmosis refers to disease caused by the intracellular protozoan parasite Toxoplasma gondii and is differentiated from the more common, asymptomatic infection caused by this organism. T. gondii is a ubiquitous parasite that infects many different types of animals, including mammals and birds.
T. gondii is classified as a coccidian and exists in three forms: tachyzoite, cyst, and oocyst. Tachyzoites, found during acute infection, are the invasive form capable of initial infection and are capable of invading all mammalian cells except non-nucleated erythrocytes. Tissue cysts are formed within host cells after infection and contain multiple, up to thousands, of bradyzoites. Such cysts are the hallmark of chronic infection and are present for the life of the host. Cysts are important in transmission of infection, as they may be present in animal tissue ingested by carnivores. Oocysts represent a stage in sexual reproduction found only in the mucosal cells in the intestine of members of the cat family, from which they are subsequently excreted in the feces.
Transmission of infection to susceptible hosts occurs by ingestion of cysts or oocysts. Cysts are present in approximately 10% of lamb and 25% of pork used for human consumption. Direct contact with any material contaminated by infected cat feces may result in ingestion of oocysts, and this form can be transmitted to food by insects. Infections may also be acquired through blood or leukocyte transfusion, by organ transplatnation, or by transplacental transmission during pregnancy. Prevalence of infection in humans varies with locale and with eating and cooking habits. In the United States, approximately 5-30% of individuals 10 to 19 years old and 10-67% of individuals over 50 years old have serologic evidence of infection.
The immune response of the host primarily governs the outcome of acute infection. Both humoral and cell-mediated immunity are important. In immunecompromised individuals, the infection can be particularly severe. Toxoplasmic encephalitis is a major cause of morbidity and mortality in patients with acquired immunodeficiency syndrome (AIDS). Toxoplasma infection also has been estimated to be the cause of approximately 35% of cases of chorioretinitis in children and adults, usually as a consequence of congenital Toxoplasma infection acquired during pregnancy. Commonly, episodic flares of chorioretinitis cause destruction of irreplaceable retinal tissue and can lead to loss of vision.
Congenital transmission occurs from mothers who become infected during pregnancy. In the United States, approximately 80-90% of women in the child bearing age group are at risk. However, diagnosis of early acute-phase infection in the mother must be distinguished from chronic infection (which does not lead to transplacental infection of the fetus) because of side effects of existing treatments. For example, pyrimethamine is known to have the potential to cause birth defects.
The most widely used tests to establish the diagnosis of acute Toxoplasma infection are the Sabine-Feldman dye test, indirect fluorescent antibody (IFA) test, indirect hemagglutination (IHA) test, and direct agglutination test. A diagnosis of recent acute acquired infection is confirmed if there is a serial twotube rise in titer when serums drawn at 3-week intervals are run at the same time or if there is a serial twotube rise in titer when serums drawn at 3-week intervals are run at the same time or if there is a seroconversion from a negative to a positive titer. A single high titer in any test does not prove the presence of active infection because of the presistent levels of IgG antibodies that occur in chronic infections. It is also possible to specifically test for the presence of IgM antibodies to diagnose early acute stage infection. However, current techniques that rely on the detection of IgM Toxoplasma antibodies require a step of distinquishing IgM antibodies from IgG antibodies, which complicates detection of the acute stage. In addition, IgM antibody levels can remain high in some cases of chronic infection for a year or more. A publication has recently reported a difference in agglutination of acetone-treated Toxoplasma parasites by patient sera as opposed to agglutination of formalin-treated parasites. However, comparison of titers of the two agglutinations is required for practical diagnosis of the stage of Toxoplasma infection.
Accordingly, simplified techniques that enable detection of the acute stage of Toxoplasma infection are needed.